Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details)

v3.6.0.2
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]    
Minimum participants percentage 15.00%  
Defined Benefit Plan, Accumulated Benefit Obligation $ 922,000,000 $ 898,900,000
Reserve for investment commitments $ 37,900,000  
Structured Finance [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Redemption request notice period, days 90 days  
Real Estate [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Redemption request notice period, days 45 days  
Withdrawal request notice period, days 65 days  
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Decrease in Projected Benefit Obligation due to Market Conditions $ 22,000,000  
Reduction in net periodic benefit expense 8,200,000  
Increase in Projected Benefit Obligation due to new mortality tables $ 13,100,000  
Percentage Increase in Projected Benefit Obligation due to new mortality tables 1.40%  
Participant contributions $ 0 0
Other Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Decrease in Projected Benefit Obligation due to Market Conditions 8,600,000  
Reduction in net periodic benefit expense 1,800,000  
Increase in Projected Benefit Obligation due to new mortality tables $ 4,900,000  
Percentage Increase in Projected Benefit Obligation due to new mortality tables 2.00%  
Participant contributions $ 6,000,000 $ 4,000,000
Tilden, Empire, Hibbing and United Taconite [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of total workforce covered by labor contract 80.00%  
Prior To Age65 [Member] | Defined Benefit Postretirement Health Coverage [Member] | United States [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Annual limit on medical coverage for each participant $ 7,000  
Prior To Age65 [Member] | Minimum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Annual limit on medical coverage for each participant 4,020  
Prior To Age65 [Member] | Maximum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Annual limit on medical coverage for each participant $ 4,500